2.1 What changes have been observed so far in climate?

Since the Third Assessment Report (TAR), improvements in terms
of data, geographical coverage, understanding of uncertainties,
and variety of measurements have allowed for a better
understanding of how climate is changing in space and time. The
warming of global climate is unequivocal and is evidenced by
numerous observations of increasing air and ocean temperatures,
widespread melting of snow and ice, and rising global average
sea level
(see Figure SPM-3).

Eleven of the last twelve years (1995 -2006) rank among the 12
warmest years ever recorded since global surface temperatures
are measured (1850). Over the last 100 years, (1906–2005) there
has been an increase in surface temperature of 0.74°C, which is
larger than the 0.6°C increase given in the TAR for the
1901-2000 period. And the warming trend over the last 50 years
(0.13°C per decade) is nearly twice that for the last 100 years.
Temperatures in the higher
atmosphere and in the
oceans (to depths of at least 3000m) have also been rising,
along with the water vapor
content of the atmosphere. Mountain
glaciers, snow cover and
ice caps have declined on
average in both hemispheres, contributing in part to the rise of
global sea level. The Greenland and Antarctic
ice sheets have also
contributed to the observed rise of sea level, which amounted to
17cm in total over the course of the 20th century.

At continental, regional, and ocean basin scales, numerous
long-term changes in climate have been observed. These include
changes in Arctic temperatures and ice, widespread changes in
precipitation amounts, ocean
salinity, wind patterns and
aspects of extreme weather including droughts, heavy
precipitation, heat waves and the intensity of tropical cyclones
(Table SPM-1).
Some aspects of climate have not been observed to change. The
difference of temperature between day and night, for example,
has remained the same, since daytime and nighttime temperatures
have risen by the same amount. Contrary to
sea ice in the Arctic,
there has been no significant decrease in sea ice in Antarctica,
which fits with the lack of observed warming in
Antarctica.
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2.2 How has climate changed in the past?

Studies of past climate have allowed inferences to be made
about past changes in global climate on time scales ranging from
a few decades to millions of years. The uncertainties related to
these conclusions on past climate generally increase with time
into the past.

This information on past climate show that the overall
temperatures of the last half century is unusual in at least the
previous 1300 years. The last time that the climate was
significantly warmer than now for an extended period (about
125,000 years ago, during the last interglacial period),
reductions in polar ice volume led to 4 to 6 metres of sea level
rise.
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2.3 What is causing the present-day changes in climate?

It is very likely that a significant part of temperature
variability in the Northern hemisphere during the past seven
centuries prior to 1950 is due to volcanic eruptions and changes
in the intensity of solar radiation. However, most of the
observed increase in global temperature since then is very
likely due to the observed increase in atmosphericgreenhouse gas
concentrations due to human activities. Human activities now
clearly affect other aspects of climate, including ocean
warming, continental-average temperatures, temperature extremes
and wind patterns.

It is likely that these increases in
greenhouse gas
concentrations alone would have caused more warming than
observed, had volcanic and human-induced
aerosols not offset some of
the warming that would otherwise have taken place.

Current climate models that simulate the observed temperature
evolution on each of six continents provide stronger evidence of
human influence on climate than was available in the 2001 Third
Assessment Report (TAR). Difficulties remain in simulating
temperature changes at smaller scales, where natural climate
variability is larger and makes it harder to estimate the
current and future impact of
greenhouse gas increases
due to human activities.
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